UCMP MYSTERY FOSSIL #42 is . . .
the dicynodont Dicynodon grimbeeki

Dicynodonts were therapsids, descendants of pelycosaurs like Dimetrodon, and ancestors of modern mammals. Therapsids varied greatly in size and came in both carnivorous and herbivorous models. Dicynodonts were strictly herbivorous, having lost their cheek teeth and developing turtle-like jaws. Some specimens of Dicynodon (like this one) had short tusks in the upper jawfossils indicate that some individuals had tusks and some did not, suggesting sexual dimorphism. The UCMP site has a gap in its information on dicynodonts, but you can find out more about therapsids here.